1. Field of the Invention
The present invention relates to a heat transfer printing sheet useful for producing raised images, in particular, raised letters for the blind. More particularly, the present invention relates to a heat transfer printing sheet for producing raised images, having excellent image-transferability, capable of producing raised images which are highly readable with the fingers and highly resistant to touch reading, and to an improved process for producing raised images.
2. Background Art
To print raised letters for the blind in accordance with out-put informations from a computer, there has been conventionally adopted such a method in that raised dots readable with the fingers are formed on paper by means of embossing, for instance, by using a raised-letter printer "TP-32" manufactured by Toyo Hybrid Co., Ltd.
In order to print raised letters on paper by the above method, it is necessary to use paper which has a thickness large enough not to be broken when raised dots are formed thereon by means of embossing. Therefore, it has been impossible to print readable raised letters on thin paper having a thickness of less than 100 micrometers, such as copying paper.
Further, when raised letters are produced by embossing, concave depressions are formed on the surface of paper, opposite to the surface which is touched with the fingers to read the raised letters. For this reason, there has been a problem in that when raised letters are formed on the back surface of paper on which letters are ordinarily printed in an ink for the seeing, it becomes laborious for the seeing to read the inked letters due to the concave depressions of the raised letters formed on the other surface of the paper. (Hereinafter, those who have normal eyesight are referred t as the seeing in contrast to the blind, and those letters which are printed for the seeing are referred to as inked letters in contrast to raised letters for the blind.)
Japanese Laid-Open Patent Publication No. 333858/1992 discloses, as a means for solving these problems, a technique of producing raised letters by melt transfer of an ink film conducted by using a heating means.
However, in order to obtain readability comparable to that of conventional raised letters produced by means of embossing, the raised letters produced by the above technique are required to have a height of 300 micrometers or more which is equal to the height of embossed raised letters. In order to transfer such a thick layer from a heat transfer printing sheet by using a thermal head, it is necessary to apply an extremely large amount of energy to the substrate film side of the heat transfer printing sheet. For this reason, there has been a problem in that the substrate film tends to be broken due to such a large amount of energy applied.
In addition, it has been very difficult to apply such a large amount of energy to a thermal head for reasons of the performance thereof.
On the other hand, Japanese Laid-Open Patent Publication No. 238984/1989 or the like discloses a technique in which heat is applied to a heat transfer printing sheet comprising a substrate sheet and a thermally-expandable ink layer formed thereon, containing an expanding agent, to transfer image-wise the thermally-expandable ink layer to an image-receiving sheet, and heat is further applied to the thermally-expandable ink layer on the image-receiving sheet so as to expand it to obtain a three-dimensional raised image. According to this publication, the thermally-expandable ink layer comprises a wax such as carnauba wax, and a thermally-decomposable expanding agent such as sodium bicarbonate or azobisisobutyronitrile.
However, those raised letters for the blind which are produced by using the heat transfer printing sheet of the above-described prior technique have the problem of resistance to touch reading; for example, due to the friction caused by the fingers, the raised letters are broken, the height thereof is readily decreased (the raised letters are worn out), or a part of or all of the dot elements of the raised letters fall off the image-receiving sheet. In addition, since the touch of these raised letters is largely different from that of ordinary raised letters formed on thick paper by means of embossing, the raised letters produced by this technique are poor in readability.
Furthermore, in the case where raised letters are produced by the use of the above-described heat transfer printing sheet, such a trouble tends to be caused that predetermined images cannot be precisely obtained, that is, the part of the thermally-expandable ink layer to which thermal energy has been applied is not fully transferred to an image-receiving sheet, or even the part of the ink layer to which thermal energy has not been applied is transferred to an image-receiving sheet, because the adhesion between the substrate sheet and the thermally-expandable ink layer is not proper.
When any excess or deficiency is present in the transferred dot elements of the raised letters for the blind, a serious problem will be brought about because such raised letters are misread even if the excess or deficiency is slight and not a problem for the seeing at all.
According to the above-mentioned Japanese Laid-Open Patent Publication No. 238984/1989, the expansion of the thermally-expandable ink layer which has been transferred image-wise to the image-receiving sheet is conducted by heating the non-printing surface of the image-receiving sheet for one minute by using a heating roller whose surface temperature is 150.degree. C.
However, in this method, the thermal expansion gradually proceeds from the interface between the thermally-expandable ink layer transferred and the image-receiving sheet towards the surface of the ink Layer. For this reason, when it is tried to expand the thermally-expandable ink layer entirely, the temperature of the binder resin contained in the ink layer reaches the softening point thereof, and coagulation is caused. As a result, such a trouble tends to be caused that the thickness of the thermally-expandable ink layer is decreased or that the adhesion of the expanded image to the image-receiving sheet is drastically decreased.
On the other hand, to print inked letters beside raised letters so as to show how to read the raised letters is considered to be extremely valuable because written informations can be simultaneously provided to both the seeing and the blind.
However, in the case where a conventional raised-letter printer employing the embossing technique is used, it is practically impossible to obtain inked letters by using a printing head useful for the formation of raised letters. Therefore, besides the printing head for producing raised letters, it is necessary to use another printing head for printing inked letters. When these two different types of printing heads are used to produce the two types of images, the process for printing the images becomes extremely complicated. Much labor and a long time have thus been needed to produce a sheet of print on which these images are printed.
Japanese Laid-Open Patent Publication No. 167156/1981 discloses, as a means for solving the aforementioned problems, a technique of simultaneously producing raised letters and inked letters by an electrophotographic process in which both an expandable toner and an ordinary toner are used.
However, in the case where a document containing both raised letters based on the method of writing Japanese raised letters, and inked letters showing how to read the raised letters is prepared by the combination use of an expandable toner and an ordinary toner in accordance with the invention disclosed in the above publication, although the inked letters can be read without difficulty, the raised letters have the problem of resistance to touch reading; for instance, due to the friction caused by the fingers, the raised letters are broken, the height thereof is readily decreased (the raised letters are worn out), or a part of or all of the dot elements of the raised letters fall off the image-receiving sheet.
In addition, the touch of the raised letters produced by this method is greatly different from that of ordinary raised letters produced on thick paper by means of embossing. Therefore, there has been a problem in that the raised letters obtained by this method is poor in readability.
The present invention is directed to overcome or at least to mitigate the aforementioned drawbacks in the prior art.
An object of the present invention is therefore to provide a heat transfer printing sheet capable of producing highly raised images which are excellent in readablity with the fingers and in resistance to touch reading.
Another object of the present invention is to provide a heat transfer printing sheet having improved image-transferability.
A further object of the present invention is to provide a heat transfer printing sheet capable of simultaneously producing inked letters, and highly raised letters which are excellent in resistance to touch reading.
A still further object of the present invention is to provide an improved process for producing raised images, for use in a heat transfer printing system, capable of producing, without damaging an image-receiving sheet, raised images which are improved in height and resistance to touch reading.
Other objects and the effects of the present invention will become apparent to those skilled in the art in the course of the following description.